CD42b GPIb╬▒ (platelet glycoprotein), glycocalicin
Molecule TypeAntigen ExpressionMolecular Weight
Min / Max
Lineage Restricted Molecule
Type 1 glycoprotein
Platelet
Megakaryocyte
145 / 145
160 / 160

Expression
CD42b is restricted on platelets and megakaryocytes.  Red blood cells, granulocytes, T cells and thymocytes do not express CD42b.  The CD42b complex is a major component of the platelet surface with 25,000 copies per platelet.


Structure
MOLECULAR FAMILY NAME:  Belongs to the mucin family.

CD42b is an a chain single-pass type-1 glycoprotein.  It contains a 481 aa N-terminal extracellular domain which has 7 24 aa tandem leucine-rich repeats (LRR) flanked by 22 aa consensus sequences, 2 hydrophilic regions, one of which is extensively O-glycosylated, 2 possible N-glycosylation sites and 3 sulfated tyrosine residues, a 29 aa transmembrane domain and an 100 aa cytoplasmic domain.  CD42b is a mucin which is part of the leucine-rich repeat family.  CD42b forms a disulfide-linked heterodimer with an a chain CD42b and b chain CD42c.  It forms a non-covalent linked complex with CD42a, cb and d.  There are several polymorphisms including the Thr 145-Met and a size polymorphism involving 1, 2, 3 or 4 copies of a 13 aa segment in the highly O-glycosylated region, each containing 5 putative O-glycosylation sites.  CD42b is rich on O-linked oligosaccharides.  The 4 repeat version is so far found exclusively in East Asian populations and the 3 repeat version in Caucasian populations.

MOLECULAR MASS
Cell Type Unreduced Reduced Comment
Platelets 160 kDa 145 kDa (+24, GPIbb)

POST-TRANSCRIPTIONAL MODIFICATION: No information.

POST-TRANSLATIONAL MODIFICATION

CD42b has 2 potential N-glycosylation sites and 1 O-glycosylation sites.  Acylation, primarily palmitoylation, of GPIb has been found.  The 3 tyrosine residues Tyr276, 278 and 279 are sulfated.

Ligands
CD42b binds to von Willebrand factor (vWf) and thrombin.  The binding site on CD42ba lies in a hinge region between the LRR motifs and the stalk.  Snake venom proteins bind to CD42b and inhibit binding of vWf.  CD42b is also a receptor for thrombin, possibly only when the CD42-GP V complex is highly multimeric as there are only 50 sites per platelet.


LIGANDS AND MOLECULES ASSOCIATED WITH CD42b
Molecule Comment
CD42c CD42b forms a disulfide-linked heterodimer with CD42c
CD42a CD42b is non-covalently linked with CD42a
CD42d CD42b is non-covalently linked with CD42d
Actin binding protein (filamin) CD42b is the binding site in the complex for actin binding protein (filamin) via the cytoplasmic region
14-3-3 z  protein CD42b is the binding site in the complex for 14-3-3 z   protein via the C-terminal region of the cytoplasmic region




Function
CD42b combines with CD42a, CD42c and CD42d to form the CD42 complex.  CD42b functions as a receptor for von Willebrand factor.  The complex mediates adhesion and activation of platelets to subendothelial matrices, which are exposed upon damage to the endothelium, at high shear rates.  It also amplifies the platelet response to thrombin during platelet activation where thrombin is involved.  The cytoplasmic region is associated via actin-binding protein filamin with the cytoskeleton. 

BIOCHEMICAL ACTIVITY

The CD42a-d complex serves as receptor for vWf and thrombin.  The actual binding site for vWf and thrombin lies on CD42b.

DISEASE RELEVANCE AND FUNCTION OF CD42b IN INTACT ANIMAL

Mutations of the CD42 complex leads to the Bernard-Soulier syndrome (BSS), which is a bleeding disorder characterized by thrombocytopenia and giant platelets.  So far, mutations have been found in the genes encoding CD42a, CD42b and CD42c in patients with BSS.  Mutations in the double-loop region enhancing binding to vWf have been found in platelet-type von Willebrand's disease. It is characterized by reduced plasma in vWf high multimers, normal platelets multimers, and thrombocytopenia.  The CD42b carries 1 aa polymorphism, Met145Thr, resulting in the Ko, Syp, HPA-2, alloantigen system.  CD42b mouse monoclonal antibodies are used for detection of auto- and allo-platelet antibodies with the MAIPA assay.

Comments
MOLECULAR INTERACTIONS -
PROTEINS AND DNA ELEMENTS WHICH REGULATE TRANSCRIPTION OF CD42b
Molecule Comment
GATA-1
Ets

SUBSTRATES: No information.

ENZYMES WHICH MODIFY CD42b
Molecule Comment
Calpain
Elastase
Cathepsin G
Plasminogen


Database accession numbers
AnimalPIRSWISSPROTEMGBL/GENBANK
 
HumanEntrezgene 2811P07359
Antibodies
AK2   View Reactivity
HIP1   View Reactivity

Revised June 25, 2008


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